The animated background had an animation assigned, but the layers painted at the
viewport size so the background-position changes were effectively static. Give
the ambient light layers larger paint areas and drift them slowly so the
animated option visibly moves without becoming distracting.
Reduced-motion users keep the same static background.
Test Plan:
- git diff --check
Refs: user redesign nitpick about animated background not moving
ddidderr
2af55981c3
